1. Field of the Invention
The present invention relates to a transmission/reception apparatus used for digital mobile communications, and particularly relates to an OFDM (Orthogonal Frequency Division Multiplexing) transmission/reception apparatus and OFDM transmission/reception method used for OFDM-based mobile communications.
2. Description of the Related Art
In an OFDM-based mobile communication, transmission data is converted to a plurality of slower parallel signals, which are superimposed over their respective subcarriers and transmitted. The following is an explanation of a conventional OFDM transmission/reception apparatus with reference to FIG. 1 and FIG. 2. FIG. 1 is a block diagram showing a configuration of a transmission system of a conventional OFDM transmission/reception apparatus. FIG. 2 is a block diagram showing a configuration of a reception system of the conventional OFDM transmission/reception apparatus. For simplification, suppose a plurality of signal lines are also expressed with a single arrow.
In FIG. 1, modulation section 11 modulates transmission data. Inverse Fast Fourier Transform (hereinafter referred to as “IFFT”) section 12 performs IFFT processing on the modulated transmission data. Transmission section 13 performs transmission processing on the IFFT-processed transmission data and transmits this data from antenna 14.
In FIG. 2, antenna 21 receives a radio signal. Reception section 22 performs reception processing on the signal received from antenna 21 (reception signal). Timing control section 23 controls each section according to a symbol synchronization timing acquired from reception section 22. Fast Fourier Transform (hereinafter referred to as “IFFT”) section 24 performs FFT processing on the reception signal. Regarding the acquisition of symbol synchronization timing in reception section 22, a variety of methods are proposed. However, their detailed explanations are omitted here.
Synchronization detection section 25 performs synchronization detection processing on the FFT-processed reception signal and removes influences of phase rotation and amplitude variation caused by fading, etc. from the reception signal above.
Thus, the conventional OFDM transmission/reception apparatus achieves large-capacity and high-quality radio communications with excellent multi-path adaptability through data transmission/reception using a plurality of subcarriers.
However, in the conventional OFDM transmission/reception apparatus, important information used for communication control such as control information and retransmission information (hereinafter, simply referred to as “important information”) is multiplied by a carrier frequency signal together with user data and transmitted through one of subcarriers.
Therefore, if the reception quality of subcarriers carrying the important information deteriorates in an extreme manner due to influences of fading, etc., errors may occur in bits representing control information and retransmission information, which will prevent appropriate control and retransmission requests, etc., extremely deteriorating the channel quality.